We have encountered a number of patients with a bleeding disorder who have a prolonged activated partial thromboplastin time. These patients appear to be deficient in a hitherto unrecognized clotting factor referred to as the Passovoy factor. Using DEAE chromatography and other separation procedures we are purifying from normal plasma a fraction that corrects Passovoy deficient plasma. We propose to biochemically characterize this clotting factor.